Parallax Sections: How To Control Size Using CSS

I want to set the height for ALL parallax sections so that they take about 80% of the vertical screen space. The other 20% is taken by the header and a call for action section (10% for each section).

I adjusted the height of the cfa-section per another support ticket and now I need to be able to set the vertical height of the parallax sections based on desktop screen height. What I want to do is have a parallax image (i.e., background-small) that takes up 80% of the vertical screen space alternating with a call for action section that takes up 10% of the vertical screen space (for desktop).

Thanks in advance!